Viscoelastic property measurements made at the solid liquid interface are key to characterizing materials for a variety of biological and industrial applications. Further, nanostructured materials require nanoscale measurements. Here, material loss tangents (tan delta) were extracted from confounding liquid effects in nanoscale contact resonance force microscopy (CR-FM), an atomic force microscope based technique for observing mechanical properties of surfaces. Obtaining reliable CR-FM viscoelastic measurements in liquid is complicated by two effects. First, in liquid, spurious signals arise during cantilever excitation. Second, it is challenging to separate changes to cantilever behavior due to the sample from changes due to environmental damping and added mass effects. We overcame these challenges by applying photothermal cantilever excitation in multiple resonance modes and a predictive model for the hydrodynamic effects. We demonstrated quantitative, nanoscale viscoelastic CR-FM measurements of polymers at the solid liquid interface. The technique is demonstrated on a point-by-point basis on polymer samples and while imaging in contact mode on a fixed plant cell wall. Values of tan delta for measurements made in water agreed with the values for measurements in air for some experimental conditions on polystyrene and for all examined conditions on polypropylene.